I. Field of the Invention
The present invention relates to determining the position of a mobile station; more specifically, to locating a mobile station using time difference of arrival (TDOA).
II. Description of the Related Art
A global positioning system (GPS) is commonly used to provide a receiver with accurate measurements of its location. The GPS receiver receives a signal from satellites and determines its positions by performing TDOA calculations based on the known position of the satellites. The receiver is generally attached to a vehicle or boat and is provided for this single purpose. The expense of the GPS receivers has generally limited its purchasers to luxury vehicle, aircraft, and boat owners.
Digital cellular/PCS phones have become a very convenient and inexpensive way for a person to communicate with other persons or communication systems from wherever the person is located. The person can also call 9-1-1 in the event of an emergency. However, to date, wireless communication systems can not accurately determine the location of the caller without the use of satellites and GPS.
Current wireless communication systems use multiple access techniques to combine signals from different sources to permit many users to share a common medium without mutual interference. One of the basic types of multiple access techniques is code division multiple access (CDMA). In CDMA, each base station transmits a pilot channel signal, which is essentially an unmodulated pseudo-random noise (PN) sequence. The PN sequence comprises a sequence of PN chips, and each PN chip corresponds to a distance of about 800.4 feet. Each base station transmits the pilot channel signal using a different timing offset such that mobile stations can distinguish from which base station a pilot channel signal was transmitted.
The mobile station is time synchronized with a serving base station, i.e., the base station in which the mobile station is in communication. The mobile searches time intervals referred to as search windows for the pilot channel signals. Each base station is configured to transmit its pilot channel signal such that mobile stations can expect to begin receiving no more than one pilot channel signal within each search window. When the mobile station detects a pilot channel signal, it measures the pilot channel signal strength and records the phase of the pilot channel signal, in terms of PN chips, as the pilot channel signal arrives at the mobile station. If the pilot channel signal strength exceeds a predetermined threshold, then the base station that transmitted the pilot channel signal is xe2x80x9cvisiblexe2x80x9d to the mobile station. The measurements and recordings are transmitted from the mobile station to the serving base station or some other predetermined location over a reverse link.
Conventional methods of determining a mobile station""s geolocation generally require an indication of distances between at least three xe2x80x9cvisiblexe2x80x9d base stations and the mobile station. The distance between a base station and a mobile station is equal to the time xcex94ti for a signal to travel from the base station to the mobile station, multiplied by a wave speed "ugr" of the signal. If xcex94t1"ugr" is a distance from the mobile station (having geographic coordinates (x0,y0)) to a first base station (having known geographic coordinates (x1,y1)), xcex94t2"ugr" is a distance from the mobile station to a second base station (having known geographic coordinates (x2,y2)), and xcex94t3"ugr" is a distance from the mobile station to a third base station (having known geographic coordinates (x3,y3)), then based on the Pythagorean theorem, the following equations can be derived for a time of arrival (TOA) approach:                                           Δ            ⁢                          xe2x80x83                        ⁢                          t              1                        ⁢            v                    =                                                                      (                                                            x                      1                                        -                                          x                      0                                                        )                                2                            +                                                (                                                            y                      1                                        -                                          y                      0                                                        )                                2                                                    ,                            (        1        )                                                      Δ            ⁢                          xe2x80x83                        ⁢                          t              2                        ⁢            v                    =                                                                      (                                                            x                      2                                        -                                          x                      0                                                        )                                2                            +                                                (                                                            y                      2                                        -                                          y                      0                                                        )                                2                                                    ,                            (        2        )                                                      Δ            ⁢                          xe2x80x83                        ⁢                          t              3                        ⁢            v                    =                                                                      (                                                            x                      3                                        -                                          x                      0                                                        )                                2                            +                                                (                                                            y                      3                                        -                                          y                      0                                                        )                                2                                                    ,                            (        3        )            
to determine the mobile position (x0,y0). However, in CDMA, the time xcex94ti is unknown because mobile stations have no absolute time reference to measure xcex94ti.
A TDOA approach reduces the number of equations from three to two (equation (3) minus equation (1) and equation (2) minus equation (1)). The TDOA approach provides accurate location determinations if no system measurement errors or multi-path effects, described below, are present. Unfortunately, system measurement errors and multi-path effects generally exist and cause deviations from true location determinations. Therefore the above equations cannot be used directly to accurately determine the mobile station M""s geolocation.
The present invention addresses these problems by providing a local positioning system (LPS) designed to use radio propagation parameters in code-division multiple access (CDMA) forward links or time-division multiple access (TDMA) reverse links to estimate a mobile station""s position.
The LPS determines the position of the mobile using triangulation methods by minimizing two set of equations, called cost functions. The first set of cost functions represent distance errors from the xe2x80x9cvisiblexe2x80x9d base stations to the mobile station, and the second set of cost functions represents position errors in the location estimation of the mobile station. Both sets of cost functions include variables common to more than one of the cost functions within the set. The cost functions are minimized by estimating values for the unknown variables within each equation so that the distance or position errors in the set are as close to zero as possible.
To determine the geographical coordinates of a mobile station when the distances between the mobile station and the base stations are not known, the LPS first estimates the distance from the mobile station to the base stations to mitigate the system measurement errors and multi-path effect. After the distances are estimated, the LPS estimates the geographic coordinates of the mobile station (x0,y0), based on the estimated distances.
In a preferred embodiment, the LPS is a software implementation on a computer to determine the geographic location (geolocation) of a mobile station. The LPS receives a data sample including information indicating arrival times of pilot channel signals at a mobile station and accesses base station information indicating the location of at least three cellular or PCS base stations to which the arrival time information is associated. The LPS then estimates the distances from the mobile station to the base stations by minimizing a first set of equations or cost functions and estimates the geolocation of the mobile station by minimizing a second set of equations or cost functions based on the estimated distances.
The LPS of the present invention provides the benefit of using existing equipment to provide GPS-like positioning capabilities. The LPS requires no additional signal detection capabilities, and only requires a minor modification to the existing wireless telephone systems. No additional hardware is needed other than the standard CDMA/TDMA system, making the LPS cost effective. The LPS can also solve the 9-1-1 mobile location problem for wireless CDMA/TDMA systems. Therefore, the LPS can determine the position of a person in distress from their digital phone.